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Abstract

We propose and demonstrate a self-referencing alignment technique to conveniently enlarge fabricated grating area. The latent image gratings are used as the reference objects to align (adjust and lock) the attitude and position of the substrate relative to the exposure beams between and during consecutive exposures. The adjustment system and the fringe-locking system are combined into the exposure system, eliminating the drift errors between them and making the whole system low-cost and compact. For the fabricated 1 × 4 mosaics of 50 × (30 + 30 + 30 + 30) mm2 area and 1 × 2 mosaics of 90 × (80 + 80) mm2 area, the typical peak-valley −1st-order wavefront errors measured by a 100-mm-diameter interferometer are not more than 0.06 λ and 0.09 λ, respectively.

Mosaic steps, including projections of relevant elements in the directions of beams B1 and B2 onto the grating substrate G and the L fringes. (a) Step I, the first exposure. (b) Step II, the recording of the reference L1II fringes. (c) The recorded reference L1II fringes. (d) Step III, the second exposure with the real-time L1III fringes locked to the reference L1II fringes. (e) Step IV, after moving G for monitoring the L1IV and L2IV fringes simultaneously, the recording of the reference L2IV fringes with the real-time L1IV fringes locked to the reference L1II fringes. (f) The recorded reference L2IV fringes. (g) Step V, after moving G, the third exposure with the real-time L2V fringes locked to the reference L2IV fringes. (h) Completed exposure areas and their groove position. In half image pairs of L fringes the arrows indicate the borders between the reference and real-time fringes. On the surface of G both the exposure fringes and the latent grating are partly plotted, to clearly show the phase difference between them.

The −1st-order interferograms and wavefronts of (a) a 1 × 2 mosaic grating and (b) a 1 × 4 mosaic grating. The peak-valley and root-mean-square errors in (a) are 0.087 λ and 0.018 λ, respectively; the ones in (b) are 0.060 λ and 0.012 λ, respectively.